Biochemical studies of mammaliam skeletal muscle have found that the major contractile protein myosin undergoes transitions in its subunit structure during development from its embryonic to its adult forms. The present project will investigate the physiological function(s) of these various forms, or isoenzymes, of myosin in rabbit skeletal muscle. This will be done through the use of single fiber preparations from which the surface membranes have been removed, thereby allowing direct control of muscle activation with Ca2+. Measurements of tension, stiffness and shortening velocity, as well as the tension transients in response to rapid length changes, will be made in order to determine the mechanical properties of the myosin cross-bridges and the kinetics of interaction of myosin with actin. The myosin isoenzyme compositions of these same fiber preparations will be determined using gel electrophoretic techniques and histochemical staining procedures that have been adapted for use on a micro scale. These mechanical and biochemical measurements will be done on muscles that are destined to be fast and those that are distined to be slow in the adult, and will encompass the embryonic, fetal, neonatal and adult stages of development. Such measurements made on the same single fiber preparations will allow straightforward conclusions regaridng the possible relationships between the myosin isoenzymes that are present and physiological function. This experimental approach will provide new and valuable information about the characteristics of the actin-myosin interaction in preparations which retain the structural organization of living muscles, and should therefore further our understanding of the molecular mechansism of muscle contraction.